Indicator Species


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•  An indicator species is an organism used to assess a specific environmental condition

Indicators species are sensitive to specific environmental conditions and consequently have a limited range of tolerance

  • Their population growth or reduction indicates changes in the environment, making them a useful means of monitoring change

Indicator species may be sensitive to a number of different environmental conditions:

  • Lichen, along with mosses, are susceptible to air-borne pollutants dissolved in water (e.g. sulfur dioxide)
  • Tubifex worms are sensitive to concentrations of heavy metals
  • Mayfly larva and certain aquatic invertebrates are sensitive to dissolved oxygen levels in water

Indicator Species

indicator species


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•  Relative numbers of an indicator species can be used to calculate the value of a biotic index

Biotic indices compare the relative frequency of indicator species and provide an overall assessment of environmental health

  • Calculating a biotic index involves multiplying the population size of each indicator species by its pollution tolerance rating

biotic index

The following conclusions can be drawn from a biotic index:

  • A high biotic index indicates the presence of many pollution-sensitive organisms, denoting an unpolluted environment
  • A low biotic index indicates a polluted environment, due to a relative abundance of pollution-tolerant organisms
  • A change in the biotic index over time marks a change in the environmental conditions within a given ecosystem

Pollution Tolerance and Environmental Pollution Levels

tolerance rating

Practice Question

Calculate the biotic index to compare levels of pollution at two locations along a stream
Site 1: 8 stonefly nymph, 1 freshwater shrimp and 1 tubifex worm was collected.
Site 2: 4 stonefly nymph, 10 freshwater shrimp and 6 tubifex worms were collected.